The present invention relates to a process for efficiently producing polypropylene sheets or films which are of high transparency although not stretched and thus which are suitable for wrapping of foodstuffs, medicines and so forth.
Sheets or films made of crystalline thermoplastic resins, particularly polypropylene, are varied in physical properties such as transparency depending on the crystal form thereof. Heretofore, polypropylene has been limited in its use because when molded in the form of sheet or sheet-molded container, it is poor in transparency, although it is excellent in physical properties such as heat resistance, moisture resistance, light weight, oil resistance and non-polluting properties.
In order to make a polypropylene sheet transparent, it is necessary to impart gloss to both surfaces of the sheet and furthermore to make the inside of the sheet transparent. As techniques of imparting gloss to both surfaces of the sheet, a method of transferring the specular surface of roll, a method of transferring the specular surface by the use of a metallic, specular finished endless belt, and so forth are known. As techniques of making the sheet inside transparent, a method of preventing crystallization in the inside by using a specific material, a method of preventing crystallization by chilling, a method of breaking crystals by stretching or orientating, and so forth are known.
Therefore, various methods combining the above techniques have heretofore been proposed. For example, a method in which the specular surface is transferred and moreover a nucleating agent is added to the resin as the starting material, is proposed in order to make the sheet inside transparent while at the same time imparting gloss to both the surfaces of the resulting sheet. In accordance with this method, however, it is difficult to produce a thin sheet, and the transparency of the sheet is not satisfactorily high.
In addition, there is proposed a method in which the specular surface is transferred and moreover a rolling treatment is applied. This method, however, has a problem that thermal moldability is poor because stretching or orientation is conducted by rolling.
Moreover, a transferring method using a metallic, mirror finished endless belt is disclosed in Japanese Patent Publication Nos. 24977/1986 and 32572/1980, Japanese Patent Application Laid-Open No. 284719/1987, etc. In this method, however, transparency is not sufficiently high.
On the other hand, various techniques of controlling crystallization by rapid chilling have been proposed to improve transparency of crystalline thermoplastic resin sheets or films, as described in Japanese Patent Publication Nos. 41457/1987 and 10816/1987, Japanese Patent Application Laid-Open Nos. 42822/1987 and 236719/1985, etc., and they have been put into practical use.
In recent years, however, sheet products have been required to have increased quality and productivity. In accordance with the conventional method, when the thickness of the sheet is more than 0.6 mm, the limit of conduction of heat in the thickness direction causes a problem that the transparency is decreased abruptly. Thus, sheets which are highly transparent although of high thickness have been desired. Moreover, from a viewpoint of increase of productivity, it is required that the sheet can be produced at a high speed.